Boronizing bath and method



United States Patent 3,201,285 BORONIZING BATH AND METHOD Vernon L. Hilland Thomas F. Stapleton, Indianapolis, Ind, assignors to General MotorsCorporation, Detroit, Mich., a corporation of Delaware No Drawing. FiledMar. 15, 1962, Ser. No. 180,015 5 Claims. (Cl. 148-611) This inventionrelates to the heat treatment of metals and more particularly to thecase hardening of metals by impregnation of metal surfaces with boron.

It is an object of this invention to provide an economical and simpleimmersion, or nonelectrolyti-c, boronizing process. It is also an objectof the invention to provide a bath composition for the boronizing ofmetals. It is a further object of the invention to provide a method ofmaking a bath useful in boronizing of metals.

Other objects, features and advantages of the invention will become moreapparent from the following description of preferred embodimentsthereof. The objects of our invention are attained by dissolvingmetallic boron in a molten bath to provide dissolved free boron in thebath. A metal workpiece is immersed in the bath and maintained thereinat a suitable boronizing temperature for a sufficient duration to obtainthe desired degree of boron impregnation. When this impregnation hasbeen obtained, the workpiece is withdraw from the bath and cooled. Ifdesired, it can then be rinsed to prepare it for any further treatments.Among further treatments which are contemplated for the invention is asubsequent heat treatment which is used to diffuse the boron furtherinto the surface of the part and, concurrently, reduce the concentrationof boron in those areas of the part closely adjacent its surface. Thisdiffusion treatment can be accomplished immediately after boronimpregnation, after cooling or after rinsing.

The physical state of the elemental boron used is not material to theinvention and any available form can be employed.

The preferred solvent for our boronizing bath is one which will dissolveboron and which will form a molten solution with the boron at atemperature suitable for diffusion of boron into the metal surface whichis being treated. The term dissolved is also to include dispersed, as itappears that, particularly in the more highly concentrated baths, boronmay also be dispersed in the bath. Molten alkalis, such as fused sodiumhydroxide and potassium hydroxide, can be used as solvents in our bathwhen dissolving boron. Of course, the baths thus formed are extremelycorrosive to most metals and are, therefore, of only limited use forboronizing. On the other hand, fused borates, including fused boricacid, are of considerably more importance as base materials for ourbath. Baths formed with borate salts in which elemental boron has beendissolved are generally satisfactory for boronizing ferrous base alloys,nickel base alloys, cobalt base alloys, molybdenum base alloys, tungstenbase alloys and tantalum base alloys. In referring to an alloy of agiven base metal, e.g., ferrous base alloys, we mean to include allmetals which contain at least 50%, by weight, of the stated base metal.Thus, the pure base metal, e.g., pure iron is also included within thescope of the term.

Boronizing baths can be formed by dissolving metallic boron in a basematerial formed with one or more of the following borates: boric acid, ametaborate salt, a tetraborate salt, a pentaborate salt and a fluoboratesalt. Of the various salts, the alkali metal salts are generallypreferred, but the salts of other metals, such as the alkaline earthmetals, can also be used. More specifically, highly satisfactoryresults, particularly with respect to the alloys previously referred tocan be obtained with sodium tetraborate. It is to be understood thatWhile fused fluoborate baths containing dissolved metallic boron areeffective boronizing baths, such baths are not generally preferred forcommercial applications. After approximately 48 hours at a comparativelyhigh operating temperature, the fluoborate baths tend to exhibit anundesirable degree of solvent decomposition. It is further understoodthat, although in many instances we prefer to use a single compound as,for example, sodium tetraborate as the base material, or solvent of thebath, our invention is not restricted thereto. Combinations ofsubstances can be used to form the base material in which the boron isdissolved and maintained at an appropriate temperature for diffusing itinto the surface of the metal being treated.

We have found that a mixture of sodium, potassium and lithiumtetraborates can not only be used up to a temperature of 2l50 F. but canbe effectively used at temperatures as low as 1250" F. While a liquidmixture can be obtained as low as 1200" F., the mixture is not veryfluid and it is, therefore, not generally preferred to use this mixbelow 1250 F. The salt mixture we employ as a base material contains 20to 60 mole percent sodium tetraborate, 20 to 60 mole percent potassiumtetraborate and 20 to 60 mole percent lithium tetraborate. Equimolarratios have been found to be highly satisfactory mixtures of thesesalts. This salt mixture not only is capable of dissolving substantialamounts of boron but, as indicated, has a high fluidity below atemperature of approximately 1500 F., where sodium tetraborate is ratherviscous. The melting point of sodium tetraborate, of course, is 1378 F.It is to be recognized that our sodium, potassium and lithiumtetraborate salt mixture is suitable for use in any process ofboronizing, electrolytic or nonelectrolytic, where sodium tetraborate,alone, can be used. However, in addition our salt mix can be used ateven a lower operating temperature.

Even when small amounts of metallic boron, about 1%, by weight, and evenlower, are used inthe bath, they can be effective in producing boronimpregnation. However, we prefer to dissolve larger amounts of boron inthe bath in order to obtain the desired diffusion in a shorter period oftime. We generally prefer to use at least about 4%, by weight, boron inthe bath so that appreciable results can be obtained Within a reasonabletime, that is, about two hours. By dissolving larger amounts of metallicboron in the bath, up to about 8% by weight, a substantial increase inthe rate of visible case depth formation can be obtained. While littleincrease in the rate of visible case depth formation is ob tained byusing more than about 8%, by weight, boron we prefer to use,particularly for commercial applications, approximately 12% boron toincrease time between boron replenishment and insure consistent results.However, while it is not economically feasible, in most instances, touse bath solutions which are saturated with metallic boron, suchsolutions provide satisfactory results and can be used.

Treatment of metal in accordance with our invention preferably involvesa precleaning of the surface which is to be treated. On the other hand,our preferred boronizing bath (one containing about 12%, by weight,metallic boron dissolved in sodium tetraborate) possesses excellentoxide dissolving characteristics. Thus, when using our preferred bath,if desired, the cleaning of the part prior to boronizing can be omitted.However, to reduce contamination of the boronizing bath and, therefore,to increase its active life, it is desirable to clean the part before itis boronized. Cleaning the metal surface in any normal and acceptedcleaning manner is suitable as a preparation for boronizing. Forexample, a ferrous base alloy can be cleaned by immersing it in asuitable oil film remover, as trichloroethylene or the like. If severelyoxidized it can also be treated in an aqueous solution containing 1%, byweight, hydrochloric acid to remove surface rust and scale.

After the part is cleaned, it is immersed in the boronizing bath, withor without a preliminary preheating step. The specific physicaldisposition of the part in the bath is no more critical to the inventionthan is the disposition of a part in any liquid bath. Thus, of course,the part should be positioned to avoid forming air pockets whichprohibit contact between the bath and any part of the surface which isto be treated. As indicated above, while, in some instances, preheatingthe part may be beneficial, it usually has little effect on thecharacter of the product obtained.

The preferred time of immersion in the boronizing bath is variable anddepends upon a plurality of factors, including the thickness of the.case desired and the rate of visible case depth formation. The rate ofvisible case depth formation for many metals in our bath diminishesrapidly after about one hour of immersion. Two hours immersion producesgenerally satisfactory results for most metals, and there is usuallylittle added benefit in using case depths greater than those formed byfour hours immersion. However, in certain instances, exceptionally deepcases and, consequently, extraordinarily long immersion times may bepreferred.

The rate of visible case depth formation is primarily dependent upon thenature of the basis metal, the bath composition and the operatingtemperature of the bath. As previously indicated, the rate of visibilecase depth formation is not only increased by increasing theconcentration of boron in the bath but also by increasing the bathoperating temperature.

The preferred operating temperature of the bath is not determinedindependently. In general, a bath formed in accordance with thisinvention is useful for boronizing at any temperature between itsmelting point temperature and its boiling point temperature, providedthat the metal treated is not adversely affected by the temperatureinvolved. On the other hand, the melting point of the metal, or the caseformed, and the presence of characteristics derived from prior heattreatments can influence the preferred boronizing temperature.Boronizing baths in which sodium tetraborate is the predominantconstituent are preferably operated at a temperature of at least 1500F., the temperature at which these baths are highly fiuid. Temperaturesin excess of about 2150 F. are to be avoided when using the sodiumtetraborate baths due to an undesirably high degree of solventevaporation which occurs at these temperatures.

It is, therefore, to be appreciated that the boronizing treatment can beaccomplished at a plurality of temperatures, durations and bathconcentrations. By dissolving larger amounts of boron in the bath, agreater visible case depth in a lesser treatment time can be obtained.Elevating the treatment temperature increases the rate of visible casedepth formation to reduce treatment duration.

The invention is applicable to a wide variety of different alloys: lowcarbon steels, such as SAE 1010 or SAE 1018, high alloy steels,including stainless steel, alloys such as SAE 310 and SAE 440, nickelbase alloys, cobalt base alloys, molybdenum base alloys, tantalum basealloys and the like. Obviously, the rate of boron diffusion is not thesame for every alloy. Hence, the treatment conditions necessary toattain a given visible case thickness and hardness may vary for onealloy from those preferred for another alloy. It appears that thegreater the proportion of alloying ingredients in a ferrous alloy, thelesser the boron penetration rate. ever, in general, the rate of boronpenetration in ferrous base alloys is larger than in cobalt base alloys.While the rate of visible case depth formation is greater in low alloysteels than in nickel base alloys, this relationship can reverse whenthe alloy content in the steel increases. 7

How-

In most instances it has been found that the rate of cooling the partafter it has been boronized has little effect upon the character of theboronized case. Thus, air cooling, water quenching or slow cooling, suchas furnace cooling, can be used, provided that the selected method ofcooling does not adversely affect previously established characteristicsof the basis metals involved.

After it is removed from the boronizing bath and cooled, the part can berinsed in a suitable solvent, such as water, to remove residual saltsthat may be adhering to its surface. The rinsed part is then ready forany further treatments which are to be performed on it.

In certain instances it may be desired to produce an unusually deepboronized case of lesser hardness. This can be accomplished by adiffusion treatment after boronizing. The time and temperature for thisdiffusion treatment, as in the boronizing treatment, are variable but,as a general rule, boronizing temperatures can be used. However, in someinstances, it may be desired to use a lower temperature. The preferredduration of the diffusion treatment generally is less than the bathimmersion time. Since the subsequent diffusion treatment concurrentlyalso produces a softer and less Wear-resistant outer surface, thesubsequent diffusion treatment is not preferred when utmost hardness andWear resistance are desired.

It is also to be understood that in some instances it may be desirableto otherwise heat treat and boronize a metal part simultaneously. Insuch instance, the preferred boronizing temperature and duration wouldalso be determined by reference to the most desirable other heattreating conditions.

In the event that it is preferred to boronize only a portion of thesurface of a part which is to be treated in accordance with theinvention, electrodeposited copper can be applied to stop ofiappropriate areas. The surface can be selectively plated with copper toleave exposed those areas which are to be boronized, or the entirety ofthe surface can be copper plated and subsequently selectively etched toexpose the basis metal in those areas which are to be boronized.

While corrosion-resistant metals, such as stainless steels, andceramics, such as aluminum oxide, can be used as a container for ourbath solution, containers of these materials are not the best for ourboronizing baths. For commercial production applications, we prefer toemploy a container formed of substantially a pure silicon carbide whichis low in silicon dioxide, as this is both thermally stable andresistant to attack by our preferred bath solution at the usualboronizing temperatures. Other silicon carbide substances, such assilicon carbide bonded with silicon nitride, are also useful.

Although this invention has been described in connection with certainspecific examples thereof, it is to be understood that no limitation isintended thereby except as defined in the appended claims.

We claim:

1. The process which comprises dissolving metallic bo ron in a moltenborate bath to provide a metallic boron concentration therein of atleast about 1%, by weight, immersing a metal part in said bath withoutelectron connection to said part, said part being of a metal into whichboron will diffuse, and continuing said immersion in said bath at aboron-diffusing temperature for a sufiicient duration to impregnate saidsurface with boron.

2. The process which comprises dissolving metallic boron in a moltenborate bath to produce a metallic boron concentration therein of atleast about 4%, by weight, to saturation, applying said bath to thesurface of a metal without electron connection to the metal, said metalbeing selected from the group consisting of ferrous base alloys, nickelbase alloys, cobalt base alloys, moybdenum base alloys, tungsten basealloys and tantalum base alloys, maintaining said bath at aboron-diffusing temperature within the range of approximately 1250" F.to 2150 F., continuing to apply said bath for approximately /5 hour to 4hours, discontinuing said bath application and thereafter heating saidpart at a temperature of approximately1200 F. to 2150 F.

3. The process which comprises dissolving metallic boron in a moltensodium tetraborate bath to produce a metallic boron concentrationtherein of at least about 4%, by weight, to saturation, applying saidbath to the surface of a metal without electron connection to the metal,said metal being selected from the group consisting of ferrous basealloys, nickel base alloys, cobalt base alloys, molybdenum base alloys,tungsten base alloys and tantalum base alloys, maintaining said bath ata boron-difiusing temperature Within the range of approximately 1500" F.to 2150 F., continuing to apply said bath for approximately A5 hour to 4hours, discontinuing said bath application and thereafter heating saidpart at a temperature of approximately 1200 F. to 2150 F.

4. A method of boronizing Which comprises fusing a mixture containingabout 20 to 60 mole percent sodium tetraborate, 20 to 60 mole percentpotassium tetraborate and 21') to 60 mole percent lithium tetraborate,dissolving metallic boron in said mixture to produce a metallic boronconcentration of at least about 1%, by weight, placing a surface of ametal in contact With said bath Without electron connection to saidmetal, said metal being selected from the group consisting of ferrousbase alloys, nickel base alloys, cobalt base alloys, molybdenum basealloys, tungsten base alloys and tantalum base alloys, and maintainingsaid contact at a boron-diffusing temperature for a sufiicient durationto impregnate said surface with said dissolved boron.

5. A boronizing bath containing sodium, potassium and lithiumtetraborates in the relative proportions of about 20 to 60 mole percentsodium tetraborate, 20 to 60 mole percent potassium tetraborate and 20to 60 mole percent lithium tetraborate, and at least about 1%, byweight, oi dissolved metallic boron.

References Cited by the Examiner UNITED STATES PATENTS 1,795,512 3 /31Schmidt et al 204-39 2,984,605 5/61 Cooper 204-39 3,924,176 3/62 Cook20439 FOREIGN PATENTS 677,113 5/39 Germany.

OTHER REFERENCES I-Ioge: Metal Progress, vol. 52, November 1947, pp.8l923.

RICHARD D. NEVlUS, Primary Examiner.

1. THE PROCESS WHICH COMPRISES DISSOLVING METALLIC BORON IN A MOLTENBORATE BATH TO PROVIDE A METALLIC BORON CONCENTRATION THEREIN OF ATLEAST ABOUT 1%, BY WEIGHT, IMMERSING A METAL PART IN SAID BATH WITHOUTELECTRON CONNECTION TO SAID PART, SAID PART BEING OIF A METAL INTO WHICHBORON WILL DIFFUSE, AND CONTINUINING SAID IMMERSION IN SAID BATH AT ABORON-DIFFUSING TEMPERATURE FOR A SUFFICIENT DURATION TO IMPREGNATE SAIDSURFACE WITH BORON.